This invention relates to vesicular imaging systems and more particularly to improved such systems having greater photospeed and archival permanence and being more convenient to use.
Vesicular imaging elements comprising a thermoplastic binder and photolytic compound which releases gas when decomposed by light are well known. Such elements are imagewise exposed, usually with ultraviolet light, then heated to expand the bubbles in the exposed areas. See, for example, U.S. Pat. No. 3,108,872. In order to fix the image and give it archival permanence photolytic compounds which lose their photosensitivity have been used, e.g., those compounds disclosed in British Pat. No. 968,466. Fixing by destroying the thermoplastic properties of the polymer is disclosed in British Pat. No. 975,456. Prior art processes generally use a photolytic compound to liberate the bubble-forming gas in the polymer matrix; they therefore give a negative image. It has been found in accordance with the present invention that a thermally decomposable gas-liberating compound in a thermoplastic photohardenable matrix without any photolytic blowing agent gives a positive vesicular image.